Work-related musculoskeletal disorders affecting the upper extremity are an important and costly national health problem. Biomechanical loading has been identified as a critical factor in the development of musculoskeletal disorders. This proposal focuses on the skeletal mechanics of the wrist and distal forearm during functional tasks. Despite its trivial sounding name, the dart thrower's motion (DTM)-wrist motion from radial extension to ulnar flexion, oblique to the anatomical axes-has been identified as one of the more important functional motions of the wrist, and is critical to tasks such as hammering and throwing. Both cadaver studies and our recent in vivo work suggest that the dart thrower's motion is accomplished with minimal motion of the bones of the proximal row of the carpus. This unique lack of motion of the proximal row during the dart thrower's motion suggests that the midcarpal joints must compensate with other unique patterns of motion, though to date this has not been studied. The ultimate goal of this proposal is to evaluate changes in the carpus during high demand tasks that are associated with workplace related musculoskeletal injury. Our first Aim is to determine the kinematics (motion and posture/conformation) of the eight carpal bones in the wrist and the distal radioulnar joint (DRUJ) during a simulated hammering task. In the second Aim we will examine this same motion in a cadaver model, which for the first time will permit direct comparison of in vivo and in vitro carpal mechanics. An especially intriguing question about the function and loading of the upper extremity is how the carpus responds to tensile loads in vivo, such as those developed when heavy objects are carried or when a wrench is used to tighten a bolt. One possibility is that the carpus is not loaded under tension as contraction of the forearm muscles offsets the tensile loads. Accordingly, in the third Aim of this proposal we will determine how tensile loading affects the carpus, with and without activation of the forearm musculature. A compressive task will also be examined in the third Aim due to the importance of pushing tasks during functional loading. The significance of this work is that it will provide heretofore unavailable in vivo data on the mechanics of the carpus and DRUJ during functional tasks. Our findings will have far reaching implications in the field of workplace ergonomics, and it will provide important basic science data that physicians and scientists can use to develop injury prevention strategies and rational rehabilitation protocols. This proposal uses advanced in vivo imaging techniques to measure, for the first time in human subjects, the effects of functional tasks and loading on the motion and conformation of the complex bony anatomy of the wrist. Understanding how the carpus and distal forearm function is crucial to reducing work-related upper extremity musculoskeletal disorders.